In some applications, it is desirable to convert an input voltage to an output voltage that is above or below the input voltage. The output may be a regulated voltage or a regulated current. As an example, it may be desirable to drive a series string of light emitting diodes (LEDs) at a regulated target current, where the voltage across the string must be about 20 volts and where the power supply may provide anywhere from 3 volts to 40 volts. In one example of a suitable converter, the input voltage is applied to a boost regulator to output a regulated boosted voltage above what would be required for the load. The boost regulator is then followed by an independently controlled buck regulator that receives the boosted voltage as an input and outputs the required regulated current for the load. The user typically sets the target output current. Such a converter requires two controller ICs (a boost controller IC and a buck controller IC), which adds cost and requires extra space on a printed circuit board. Further, since the switches in the boost side and buck side switch independently, audible beat frequencies may result. Other disadvantages exist.
Buck-boost regulators are also known that use a single controller for controlling an H-bridge of four switches (e.g., MOSFETs), where an inductor is connected across the bridge. The four switches are controlled to either act in a buck mode or a boost mode, or both, depending on whether the input voltage is detected as being above or below the required output voltage. Such converters have a switch between the input terminal and the inductor and between the output terminal and the inductor. Therefore, such converters can have a high input ripple and a high output ripple. The high input ripple adversely affects other circuits connected to the power supply bus, while the high output ripple adversely affects certain types of loads. The high ripples also generate high electromagnetic interference (EMI) at the switching frequency and its harmonics.
Organic LEDs (OLEDs) require a regulated voltage rather than a regulated current. The above-described current regulators may easily be converted to voltage regulators by feeding back an output voltage signal rather than a load current signal.
In certain applications, a converter having low ripple at the input and output is important, as well as having low EMI. Thus, what is needed is a converter for driving LEDs that may convert an input voltage to a higher or lower voltage, where only one controller IC is needed, and where there is low input and output ripple.